The present invention relates to a continuous method for surface treatment of metal strips, in which a finish coat having a coat thickness of less than 3 μm is applied to the metal strip, the apparatus employed for application being equipped with at least one IR sensor which faces a coated side of the strip, operates in reflection geometry, and serves to measure the coat thickness of the finish coat while the strip is moving.
The steel strips are generally protected from corrosion by means of appropriate measures. This generally involves multistage operations. In a first step the steel strips are coated typically with zinc or zinc alloys. The action of the zinc derives first from the fact that it is baser than steel and thus initially undergoes corrosion itself. The steel surface remains intact as long as it is still covered continuously with zinc. Moreover, in the presence of atmospheric oxygen, a thin oxide layer forms on the surface of Zn or its alloys and slows down corrosive attack on the underlying metal to a greater or lesser extent in accordance with the external conditions.
The protective effect of an oxide layer of this kind is generally boosted by subjecting the Zn surfaces to an additional passivating treatment. In the course of such a treatment, some of the metal to be protected dissolves and is incorporated at least partly into a film on the metal surface. Instead of the term “passivation coat” the terms “conversion coat”, “aftertreatment coat” or “pretreatment coat” are also used, synonymously.
The implementation of a passivation of this kind through treatment of the galvanized steel surface with acidic Cr(VI) and/or Cr(III) solutions is known. Increasingly, however, use is also being made for this purpose of chromium-free formulations, examples being phosphate formulations or else formulations which comprise different polymers.
One important class of polymers which are used in formulations for passivating is that of acidic, water-soluble polymers, such as, for example, polyacrylic acid or copolymers of acrylic acid with other monomers, more particularly with other acidic monomers such as vinylphosphonic acid, maleic acid or itaconic acid. The use of polymers of this kind for passivation is disclosed in, for example, WO 2004/074372, WO 2005/042801, WO 2006/021308, 2006/021309, WO 2006/134116, WO 2006/134117 or WO 2006/134118. Also known, however, is the use of polymers with N-containing groups, such as, for example, polymers which comprise vinylimidazole as disclosed by WO 2004/81128, for example.
Passivation coats as a general rule are very thin. The dry coat thickness of passivation coats is generally not more than 3 μm; in modern-day passivating processes, the aim is for a dry coat thickness below 1 μm, such as 0.01 to 0.4 μm, for example. For quality control and for control of the process it is necessary to determine the coat thickness.
This control is generally practiced discontinuously: in other words, in ongoing operation, a sample is cut from the moving metal strip and is subjected in the laboratory to wet-chemical or spectroscopic analysis. This process is unsatisfactory, being time-consuming and failing to allow a sufficiently rapid response to changes in strip quality.
Also known, furthermore, is the use of X-ray fluorescence analysis for monitoring the surface treatment. This does allow monitoring, but the X-ray fluorescence analysis can be used only to determine certain conductive elements, and even for such elements is unable to determine deviation in the chemical composition if the concentration of a conductive element is otherwise the same. For instance, X-ray fluorescence processes are unable, for example, to distinguish between the occupation of the surface with chromate compounds and its occupation with chromium(III) salts. Moreover, X-ray fluorescence processes cannot be used to determine thin coats with organic coating materials such as oils and polymers, for example, since the processes do not respond to these materials. Other conventional coat thickness measurement processes do not have a sufficiently high sensitivity to provide a quantitative description of coat thicknesses smaller than 1000 nm.
The use of IR spectroscopy processes in the course of the production of metal sheets and metal strips is known.
EP 488 726 A2 discloses a process for silicon steel sheets that defines the surface quality of the silicon steel via the intensity of infrared absorption bands. That process, however, is not continuous.
DE 199 41 734 B4 discloses a continuous method of process control in the pickling of steel strip. Pickling is a term used by the skilled worker to describe the removal of oxides and other contaminants from metallic surfaces. Through analysis of the intensity of reflected IR radiation, the degree of scaling of the metal surface is determined, and the pickling operation is controlled as a function of the measurement values.
DE 199 41 600 C2 discloses a method of process control and process optimization during the hot roiling of steel, in which the electromagnetic radiation emitted by the hot metal, in the form of a spectrum, is subjected to online capture and evaluation, and where there is evaluation of crystallographic transformations of the metal that take place at defined temperatures.
Our earlier application EP 08152645.1 discloses a method of continuous monitoring of the surface of metal strips bearing thin finish coats by means of a Fourier transform IR spectrometer. Using a Fourier transform IR spectrometer it is possible to measure the entire IR spectrum with the finish coat, but the spectrometer has moving parts, and so must not be exposed to any vibrations in operation.